WritableComparables can be compared to each other, typically
via Comparators. Any type which is to be used as a
key in the Hadoop Map-Reduce framework should implement this
interface.

Note that hashCode() is frequently used in Hadoop to partition
keys. It's important that your implementation of hashCode() returns the same
result across different instances of the JVM. Note also that the default
hashCode() implementation in Object does not
satisfy this property.